The invention relates to a method and a system for controlling vehicle speed in a vehicle equipped with cruise control when the vehicle is travelling downhill. The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a heavy-duty vehicle, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as articulated haulers, wheel loaders, and other working machines or automobiles.
Driver assistance systems for automatically controlling the speed of a vehicle, often termed cruise control systems, allow a driver to specify a selectable driving speed via actuating means. The set speed is then maintained independently of the operation of an accelerator pedal of the vehicle. The speed can be set by the driver and can also be retained independently of the driving situation, such as uphill or downhill driving. The vehicle will then use suitable means such as auxiliary brakes or service brakes to maintain the set speed.
DE 10 2006 051 179 A1 (corresponding to U.S. Patent App. Pub. US2007/0099579) relates to a method for automatic downward velocity control, commonly known as cruise control, which is capable of absorbing energy induced by the inclination automatically. In this way the driver must no longer be active to maintain a substantially constant traveling speed. The downward velocity control is triggered when the accelerator pedal is released, and the vehicle begins to accelerate downhill. The vehicle speed is controlled towards target vehicle speed, which is a desired predetermined constant vehicle speed set by the driver.
When travelling downhill in a vehicle with cruise control, it is not always possible to maintain the set speed by using auxiliary brakes only. Reducing the vehicle speed and/or manually resetting the set speed is often necessary to avoid over-using the service brakes in long or steep downhill conditions. Extended periods of service brake actuation can cause the brakes to overheat, which can cause the brakes to wear at an increased rate and may even damage the brakes.
In a conventional vehicle this has to be done manually by the driver either by performing a downshift or by adjusting the set speed and pressing the brake pedal to actuate the service brakes to reduce the vehicle speed down to the desired speed. Either action will automatically deactivate the cruise control system, which must then be manually re-activated and re-set by the driver to a lower speed that the auxiliary brakes can maintain. Alternatively, the driver must actively continue to support the auxiliary brakes by applying the service brakes to maintain the speed, which may cause overheating of the service brakes. At the end of the downhill road section, the set speed has to be manually adjusted by the driver back to the originally set value.
It is desirable to provide a method and a system for controlling vehicle speed in a vehicle equipped with cruise control which eliminates the above problems. The invention allows, according to an aspect thereof, automatic control of the speed of a vehicle requiring a minimum of intervention and providing a high level of comfort for the driver.
It is desirable to provide a method and a system for controlling vehicle speed in a vehicle equipped with cruise control.
In this context, the term “service brake” is used to denote the normal, or main brakes used in heavy-duty vehicles, usually operated by the driver depressing a service brake pedal. The service brakes are often, but not necessarily, air brakes operated by compressed air controlling friction brakes provided on each wheel to be braked on the vehicle. The term “auxiliary brakes” is used to denote additional means for assisting braking of the vehicle. Auxiliary brakes are used in addition to the service brakes and allow for prolonged brake application without requiring assistance from the service brakes. In this way, wear and unnecessary heating of the service brakes can be avoided. Such auxiliary brakes can comprise an exhaust valve, for activating an exhaust brake, engine brake valves, for activating a compression release engine brake, which auxiliary brakes are dependent on engine speed and gear selection. Alternative auxiliary brakes can be a fluid retarder, for generating a braking force on a propeller shaft, or means for driving a compressor or generator to generate a braking force. In the subsequent text, references to auxiliary brakes are intended to include the former type of brakes only, unless otherwise specified.
The invention is applicable to vehicles provided with an engine cruise control system. This cruise control system can be enabled by the driver prior to or during downhill travel. When the actual, or current vehicle speed exceeds a brake set speed set by the driver, an auxiliary brake such as the engine brake will engage to maintain that speed within the limits of the engine brake. The brake torque supplied by this type of auxiliary brake is directly dependent on the engine speed.
According to a first aspect of the invention, a method for controlling vehicle speed in a vehicle in a vehicle equipped with cruise control when the vehicle is travelling downhill is provided. The method comprises the steps of;                driving a vehicle downhill with the cruise control set to a first brake set speed (vS1);        detecting a current vehicle speed;        applying a brake torque using at least an auxiliary brake to maintain the first brake set speed (vS1), which brake torque is dependent on a currently engaged gear;        detecting a manual request for a downshift from a currently engaged gear to a lower gear, in order to decrease vehicle speed;        determining if a downshift is permissible at the current vehicle speed; and        
If a control unit determines that a downshift is not permissible, then the control unit is automatically arranged to perform the steps of:                applying a brake torque using at least a vehicle service brake to retard the vehicle speed        retarding the vehicle from a current, first vehicle speed to a lower, second vehicle speed at which a downshift to a lower gear is permissible;        performing a downshift to a lower gear when the second vehicle speed is reached;        setting the cruise control brake set speed to a second brake set speed (vS2) that is lower than the first cruise control brake set speed value; and        applying a brake torque using at least the auxiliary brake if a detected current vehicle speed exceeds the second brake set speed (vS2).        
An advantage of this method is that the driver is not required to perform any action apart from requesting a downshift to reduce the speed of the vehicle to a lower brake set speed. The request for a downshift can involve one or more gears, as well as a downshift to a lower range. No actual gear shifting using the gear shift lever is required and no additional switches or controls need to be actuated and no new set speed has to be selected by the driver. A reduction of the manual actions required for this purpose allows the driver to concentrate on the road and improves the driving comfort.
According to a first alternative example, the control unit is automatically arranged to set the second brake set speed value at a level where the second brake set speed can be maintained by the auxiliary brake.
According to a second alternative example, the control unit is automatically arranged to set the second brake set speed value at a level where the second brake set speed can be maintained by the auxiliary brake in combination with intermittent application of the service brakes. The control unit can be automatically arranged to prevent application of the service brakes if the service brake temperature exceeds a predetermined temperature.
The method is arranged to repeat the above steps until it is detected that the downhill condition has ended, i.e. the slope is levelling out and normal cruise control can be resumed. Hence, when travelling downhill with the speed set to the second brake set speed it is possible to perform further speed reductions if required. Should the control unit detect a further manual request for a downshift, for instance caused by a steeper than expected slope, then the brake set speed is set to a third brake set speed that is lower than the second brake set speed.
When the control unit detects that the downhill condition has ended then the cruise control brake set speed is restored to the first brake set speed and one or more upshifts are performed. According to a first alternative example these steps can be performed automatically in response to one or more signals transmitted to the control unit by an inclination sensor, an accelerometer, an on-board topographic database, a GPS-receiver, or a similar suitable device.
According to a second alternative example the cruise control brake set speed is restored to the first brake set speed and one or more upshifts are performed when the driver issues a resume command. Such a resume command can involve the driver actuating an accelerator means or a resume switch on the dashboard, or on or adjacent the steering wheel.
An advantage of the latter method steps is that the driver is not required to perform any action, or alternatively only a single action involving a brief application of the accelerator pedal or actuating one switch, to resume the first brake set speed of the vehicle. No additional switches or controls need to be actuated and no new/earlier set speed has to be selected by the driver. This further reduces the manual actions required, which allows the driver to concentrate on the road and improves the driving comfort.
According to a second aspect of the invention, a vehicle speed control system in a vehicle in a vehicle equipped with cruise control when the vehicle is travelling downhill is provided. The system comprises a sensor arranged to detect a current vehicle speed, a transmission having multiple selectable gears, a service brake, at least one auxiliary brake, having a brake torque that is dependent on a currently engaged gear, and a cruise speed control unit having a user selectable input for receiving a brake set speed, said control unit being arranged to compare the current vehicle speed with a first brake set speed and control the at least one auxiliary brake to maintain the first brake set speed.
The control unit has a user selectable input for receiving a manual request for a downshift from a currently engaged gear to a lower gear, in order to decrease vehicle speed. When a request is received the control unit is arranged to determine if a downshift is permissible at the current vehicle speed, that is, if the requested downshift can safely be performed from the currently selected gear to the requested gear at the current vehicle speed.
Should the control unit determine that a downshift is not permissible, then the control unit is arranged to automatically initiate a number of predetermined actions. First, the control unit is arranged to apply the service brake to retard the vehicle speed from the current, first vehicle speed to a lower, second vehicle speed. Second, the control unit is arranged to perform a downshift to a lower gear when the second vehicle speed is reached. Third, the control unit is arranged to set the cruise control brake set speed to a second brake set speed value that is lower than the first cruise control brake set speed value. Finally, the control unit is arranged to apply a brake torque using at least the auxiliary brake if the current vehicle speed exceeds the second brake set speed value. In the final step, the brake torque applied can be provided by the at least one auxiliary brake in combination with the service brake.
An advantage of this vehicle speed control system is that the driver is not required to perform any action apart from requesting a downshift to reduce the speed of the vehicle to a lower brake set speed. The request for a downshift can involve one or more gears, as well as a downshift to a lower range. No actual gear shifting using the gear shift lever is required and no additional switches or controls need to be actuated and no new set speed has to be selected by the driver. A reduction of the manual actions required for this purpose allows the driver to concentrate on the road and improves the driving comfort.
According to a first alternative example, the control unit is arranged to set the second brake set speed value at a level where the brake set speed can be maintained by the auxiliary brake.
According to a second alternative example, the control unit is arranged to set the second brake set speed value at a level where the brake set speed can be maintained by the auxiliary brake in combination with intermittent application of the service brakes. In this context, the control unit can be arranged to prevent application of the service brakes if the service brake temperature exceeds a predetermined temperature.
According to a third aspect of the invention, the object is achieved by a computer program comprising program code means for performing the above-mentioned method steps for controlling vehicle speed in a vehicle equipped with cruise control when said program is run on a computer. The invention further relates to a computer readable medium carrying a computer program comprising program code means for performing the above-mentioned method steps for controlling vehicle speed in a vehicle equipped with cruise control when said program product is run on a computer. Finally, the invention further relates to a control unit for controlling vehicle speed in a vehicle equipped with cruise control, the control unit being configured to perform the steps of the above-mentioned method.
By the provision of a method and a system which involve automatic control of a cruise control system, the advantage is that the driver can be relieved of a number of the manual actions by transferring control of the cruise control system to a control unit. With the cruise control system active, the driver merely needs to request a downshift to indicate that a speed reduction is desired. When the downshift has been performed, the control unit will automatically set the brake set speed to a lower value. This lower value can be retained to the end of the downhill road section, or until the driver indicates a further speed reduction. At the end of a downhill section the control unit can be arranged to automatically reset the brake set speed to the initial brake set speed. Alternatively, the driver can indicate that the initial brake set speed is to be reset by issuing a resume command, for instance by actuating a resume switch or by depressing an accelerator pedal, or simply by demanding an upshift back to the original gear engaged prior to or during downhill travel. In this way the number of manual actions required by the driver can be reduced to an absolute minimum, allowing the driver to concentrate on the road and improving the comfort for the driver.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.